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It’s my understanding pistols with rebated rim all headspace out on the end of the case so case length is important. I heard some where or read that with the 10mm and the 40.cal it was not the case. I’m starting to reload for the 10 so I want to clear this up first so I can keep the case length within spec. On a side note are pistol chambers generally over sized to keep loading and ejecting more reliable
 

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The 10mm headspaces on the mouth of the case. Most all chambers (other than custom chambers) are longer and fatter than the maximum case length and diameter. Because the firing pin protrudes up to about .060" there is some forgiveness in case length but I like my cases very close to the chamber length. That ensures consistant ignition, less jump, and maybe longer case life. The drawback, if you can call it that is that you trim after each sizing to make sure the case will seat completely without pinching the case mouth on the bullet.

Most pistol loaders don't even trim their cases so they are happy with cases that are shorter than the chamber and don't realize the risk of pressures from a pinched case mouth on the bullet. It seems to work for them but I like tight groups and consistent accuracy as much in my pistols as in my rifles.
 

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Elk Whisperer (Super Moderator)
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To truly shock yourself, measure new, unfired pistol brass (any straight wall case really) then compare it to just fired brass. It's always shorter, even after resizing. I was a bit taken aback on my first encounter

"DAAAD! What, how, why?!!???"

RJ
 

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Recoil junky,
Out of 200 once fired 9mm cases 116 cases trimmed after firing and sizing. I trim at .753" and leave my trimmer set at that length so I don't have to adjust it each time. I have spools that I use to size other cases that are cut to limit the trimming to what fits my guns safely. 58% of the cases trimmed after the first firing (I counted them) and that number increased on the second and third firing. I have had the same results with 40 S&W and a long history with my 357 Magnum and maximum. After 5 - 7 firings the cases trim less and tend to stay within .0005" of the trimmed length due to hardening of the brass. The only case I ever saw that got shorter was my 257/7x57mm brass. The length from the shoulder to the base actually shortened .002" when fired the first time. I chalked that up to the case taking on the chamber dimensions and the fact that I was fire-forming 7x57mm to 257 Roberts. I still have to fire-form another 100 cases before I clean them up and resize, check the length, and fire them a second time.

I can't explain how you can get shorter brass after resizing but I am open to believing it happened. It may be one of those reloading mysteries that I haven't run into yet.
 

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Paul,

Back around 1990, I ran a bulk purchase of Winchester 45 Auto brass through 50 reloadings. By the end, between range losses and split mouths, I had about 200 out of 500 left, and they were all a whopping 0.025" shorter than they were originally, so they were averaging a loss of half a thousandth in length per case.

The cause is using loads at pressures too low to stick the case walls to the chamber before the bullet starts moving, allowing the whole case to be pushed back against the breech end of the chamber. Sticking is necessary for stretching a case at its pressure ring because something has to hold the case body forward in the chamber while the pressure builds to the yield point of the brass to get the stretch.

The 45 Auto, like the 9 mm, has a case with tapered sides. When the case backs up the case swells out into that wider rear portion which actually shortens it a little. Then when it is resized, narrowing by the sizing die doesn't quite flow the fattened brass back forward 100%. A little brass stays flowed to the rear, subtracting the half-thousandth I mentioned.

The 357 Magnum and 9 mm are right on the border of sticking and stretching pressure, and many folks find their cases in these chambering just seem to keep their length.


Alittlefishy,

A rebated rim is one that is narrower than its case body. The 284 Winchester, for example, has one. But most common pistol cases are merely rimless, not rebated rim cases. The 10 mm is rimless and not rebated as both the rim and the datum point 0.2" forward of the breech side of the head have a 0.425" maximum dimension.
 

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Some tapered rimless cases taper (like thr 9X19)....some rimless cases are pretty clost to straight walled (.45scp/10mm..40sw..380...and a few other odd-balls).

Pretty much the LENGTH of a rimless pistol case that sets the heaspace.....but the headspace of any given chamber is made up of it's real-world length and any "Slop" from the breech locking up on the barrel.

YOu might get some "bonus points" for figuring out that the real-world chamber length/headspace length of that particular chambering. Still need to trim length, but the length might be a bit longer (or shorter) than the published case length.

Youmight5 find your shooting/your pistol can't tell the difference between min and max headspace so long as it is the SAME (wrong) headspace.
--------------
The MOASU's(mother of all screw ups) is the .38Super.
Some barrels were made to headspace from the semi-rim.
Some barrels were made to headspace from the case length.
They made two levels of power....,38 ACP and .38 Super (does that not remind you of the levels of 10mm factory ammo?).
Some of the .38acp/.38 super factory ammo was crimped and some ammo was un-crimped...which screwed up the headspacing of either type (semi-rim of case length).
So some advice for the 10mm/40SW shoioters (and .45acp/.380shooters could follow this as well).
Find out what the acutal/real-wold headspace/case length is right for your indiviual chamber.
Set the case trimmer for that lenght (minus a cdouple of 1/1000ths)
Trim all the cases to that lenght (and most of the wont ever see the case cutter doing any cutting until they're fired few times and grow).

BTW...an old NRA solution was to DEEPEN the rim-headpsacing .38 super chambers,cut the to chamber square to headpsace on the case mouth, cut/trimm .351WSL cases to the right (longer than standard) length.
OR....
Use sharp edged SWC bullets,set them up to actually heaspacpce from the bullet engravemnt,reduce loads a little,and let the SWC shelf do the headpacing.
After 4 or 5 firings lkoaded like the above, cases likely grew in length enough to actully length-headpsace on the case mouth once trimmed.
That can also work in the .40sw/10mm...at least it workd out for me taht way with my examples.
 

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UncleNick,
According to SAAMI specs the 10mm and 40 S&W have rims that are .001 to .003" smaller than the pressure ring just above the rim. I believe that could qualify those cases as rebated rims.
 

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Elk Whisperer (Super Moderator)
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UncleNick,
According to SAAMI specs the 10mm and 40 S&W have rims that are .001 to .003" smaller than the pressure ring just above the rim. I believe that could qualify those cases as rebated rims.
Uhmmm, I don't think so Tim.

10mm



40 S&W



Maybe you are measuring fired cases? From a Glock?

😱

RJ
 

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Recoil junky,
Those are not SAAMI drawings.
The drawings that SAAMI puts out show the rim of 40 S&W at .424 (-.010) and the diameter of the cartridge .200" above the rim to be .424". The maximum rim diameter is .424 but the allowed rim diameter can be .414".
The Speer manual shows the case right above the groove to be .4241 - it is the widest diameter on the tapered case.
The SAAMI drawings of the 10MM case shows it has a rim diameter of .425 (-.010) and the case is .425" diameter at .200" up from the base. Again allowing the rim diameter to be as small as .415" in diameter.
Speer also lists the 10mm case rim at .425" and .4252" on the other side of the case.
That shows that the case continues its taper to the groove.

If you like I can post the actual SAAMI drawings for the case and chamber for each cartridge.
I would never put my own measurements up as SAAMI specs I pulled the numbers I used from a manual - The Speer #13 manual to be exact. The numbers I used in this post are SAAMI spec numbers which show the largest diameter on the case is larger than the largest diameter for the rim and that the rim can be .010 smaller than the maximum and still fall into the SAAMI specifications.
In all fairness SAMMI does not call it a rebated rim and what I said was it COULD be called a rebated rim. If the definition of a rebated rim is that it is smaller in diameter than the largest diameter of the case then these two cartridges fit that description... According to SAAMI specifications.
 

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Elk Whisperer (Super Moderator)
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As you pulled your measurements from a Speed manual, I pulled mine from Lyman's 50th.

one ten thousandth of an inch (.0001" ) does not make it a rebated rim.

SAAMI drawings of both cartridges are identical to those I posted, not similar, identical. To be classified a rebated rim, the diameter immediately ahead of the extractor groove is what is larger than the base, not two tenths (1/5 or .2" ) from the base.

Some people will argue the sun up.

in all fairness of course.

RJ
 
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here is the SAAMI drawing of the 10mm:

99298
1592845127861.png


You will notice that:
1. the case diameter is taken at .2" from the base and not at the largest diameter at thr point just above the groove.
2. the diameter of the rim is no larger than the case diameter taken above the largest diameter of the case.
3. the diameter of the rim can be .415 to .425"
As you can see your drawings are not "IDENTICAL" to the SAAMI drawings.
 

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The Hog Whisperer (Administrator)
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A couple of thousandths smaller is manufacturing tolerances, to me.

Going down to the next common bolt face size - I'd call that "rebated". Your opinion may differ.
 

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The .50 Beowolf is a rebated rim. Those above are the 'same' for gunsmithing purposes.
It's bigger 'n a 9mm and smaller 'n a .45.
BUT, I thought one of the .40s had a rebated rim so it could be used in 9mm slides and frames. Which one is that?
 

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Elk Whisperer (Super Moderator)
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shooterPaul, you will notice that the head diameter and the web diameter in the saami drawings are identical to the drawings supplied by Lyman if one looks at the "mean measurements", or "as designed" the numbers are indeed identical. The - .010" may be indicating that is the minimum diameter the head can be and still expect reliable extraction.

RJ
 

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Elk Whisperer (Super Moderator)
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If my carbide 40 S&W/10mmdir sizes the web to .415, does that then make the cases rimmed?

RJ
 

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What does it matter if the chamber doesn't use a rim to headspace? Would turn them to full rebated rim...the extractor and ejector may not work,but it would headpsace any way.
 

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The drawings are not identical - the base dimensions are, but the actual drawings are not. You can't define the case style by using the tolerances, you define it by the base dimensions.

Both cases are rimless.
Both cases headspace at the mouth.

End of story and silly argument.
 

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Elk Whisperer (Super Moderator)
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The drawings are not identical - the base dimensions are, but the actual drawings are not. You can't define the case style by using the tolerances, you define it by the base dimensions.

Both cases are rimless.
Both cases headspace at the mouth.

End of story and silly argument.
That's what we've been trying to get across montmac. For the practical purposes of 99% of reloaders the drawings are the same.

Anyways

RJ
 

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I am not trying to convince anyone that the 40 and 10mm are rebated cases.
According to all the drawings the case diameter is not taken right in front of the groove. It is taken at .2" from the base of the cartridge, well in front of the groove. The case diameter in front of the groove is "undetermined" by SAAMI but in reality it is slightly larger than where the measurement is taken. That makes the case slightly larger than the maximum diameter of the rim and the minimum diameter of the rim is .010" smaller then the case dimension provided.
I treat both as rimless cartridges but I am a machinist and .01" is not a "manufacturing tolerance". It is the range of acceptable sizes allowed. It is remarkable to me that I am getting arguments over measurements that are listed as fact in the SAAMI drawings. Calm down, look at the measurement specified in the SAAMI drawings and the numbers are what they are. Don't call it a rebated case - it is not one - but the maximum rim diameter is smaller than the maximum case diameter right in front of the groove unless the case has a straight wall from the rim to .200" up from the case head. (it doesn't if you look at the Speer manual) The taper continues from the mouth to the groove and the rim is cut smaller by .0001 - .0002" for the maximum rim diameter. That area is not defined in the SAAMI drawing so it could be made straight, larger or smaller and still fit the SAAMI specification. I have never said that these cases were rebated cases. I said, if the definition of a rebated case was that the rim was a smaller diameter than the largest diameter of the case body then it could be called a rebated case. To me it is a rimless case. In either event it does headspace on the mouth of the case. We are all in agreement there (I hope). That the rim can be cut .010" smaller diameter and still fall within the SAAMI spec is plain fact - it is listed on the drawing. We can all see that.
 

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The 'taper' in a straight-walled case actually starts at the thick portion of the head which is not sized by die. You can assume the draw die draws a truly straight case and then it is sized to final dimension. Since rims and extractor grooves are variable, the .200 ahead of the base set as a starting dimension makes sense. Depending on the extractor, the rim could be cut .020 under and still function. The rim only has to fit the bolt/breech face.
I sure thought somebody had a .40 cal with a 9mm base for switch barrel autos, but maybe I just imagined it.
 
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